Parasitic oscillation elimination



Sept..19, 1944. y e. M. BROWN ,3

PARA'SITIC OSCILLATION ELIMINATION Filed May 29, 1943 INDUCT/ VE Ihventor": George M. Brown,

y} 52% His ttorneg,

Patented Sept. 19, 1944 UNI-TED STATES PA ENT OFFlCE 2,:5s,s1z

PARASITIC OSCILLATION ELIMINATION George M. Brown, Scotia, N.- Y., assignor to General Electric Company, a corporation of New York desired oscillations in'high frequency circuits; and is an improvement upon the'invention described and claimed in U. S. Letters Patent No.

2,251,629 issued on August 5, 1941 to Laurence M. Leeds.

Amplifiers and other circuits operating at high frequencies frequently include arrangements of circuit elements 'or components such that leads or conductors of substantial length must be em- ,ductance: i 3. 5

ployed; such conductors have appreciable inductance and may resonate with capacities in the circuit to produce undesired parasitic oscillations. In accordance with .the teachings ofthe aforesaid patent these undesired oscillations are efiectively damped and minimized by providing a circuit including a condenser and a resistance in series connected to shunt the conductor;' the resistance unavoidably includes inductance and.

this inductance is tuned with thecondenser to series resonance at the undesired frequency. A purely resistive path is therefore provided for current at the undesired frequency. It has been found, however, in very high power amplifiers or other circuits that the heating loss in the shunting resistor at the fundamental frequency of the apparatus may be objectionably high. Ac-

cordingly, it is an object of my invention to provide an improved high power high frequency circuit in which undesired oscillations and resistance heating losses are minimized.

, The features of my invention which I believe i6 and i'l, respectively, which are connected to ground and with anodes or plates I8 and it, re-. spectively, connected to the ends of the output in- The central point of the inductance lZis connected to ground through a choke coil 20-and a battery or other suitable source of grid bias potential if. A bypass condenser 22 is provided around the source 2!. Condensers 23 and 2,4 are connected between the ends of the inductance i2 and ground and are selected so that their capacities, together withthe unavoidable interelectrode and circuit capacities, resonate with the inductance i2 at the frequency of the input signal wave impressed on the inductance.

In the output circuit the central point of the inductance i3 is connected to ground through a choke coil 25and a suitable source of anode potential such as a battery 26, the battery being bypassed by a condenser 2?. The condensers 2B and 29 are connected between .the respective ends of the inductance l3 and ground and the capacity of these condensers, together with other capacities which are unavoidably present, are tuned with the inductance l3 to resonatevat the nequencyof the signal wave. Conventional neutralizing condensers 30 and Marc provided to neutralize the ano'de-to-grid capacity effects of the devices 10 and H. The condenser 38 is connected between the anode iii of the device ill and 'the grid ii of the device H by a conductor 32.

to be novel are set forthwithparticularity in the appended claims. My invention itself, however, both as to its organization and method of operation together with further objects andadvantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing inwhich Fig. 1 illustrates schematically a. high frequency push-pull amplifier embodying my invention, and Fig. 2 illustrates another embodiment of my in-' vention. A

The amplifier illustrated in Fig. 1 includes a pairof electron discharge devices I!) and II which are connected to operate in balanced or push-pull relation to amplify a signal or other 'high frequency wave from a source (not shown) which wave appears across an input inductance l2.- The amplified wave is impressed across an output inductance I 3 and is utilized ina suitable load circuit (not shown). The devices l0 and It include control electrodes or grids l4 and I5; respectively,- connected to the ends of the inputinductance II. The devices l0 and II are providedwith cathodes The condenser 3! is connected between the anode f9 and grid H by a conductor 33. Athigh fre-.

' quencies the length of the conductors 32 and 33 above or below the ope is substantial and suflicient inductance is'present so that it resonates with the interelectrode capacities of the devices In an I i and with other inductance andcapacity whichis present in the circult, The oscillations resulting from these resonant conditions may occur at frequencies either ting frequency of the These undjired or-parasitic oscillations may completely destroy the usefulness of the amplifier if they are sumciently strong or they may produce erratic operation or spurious radiations and in any event they reduce the useful output of the amplifier. Inaccordance with the aforesaid Leeds patent the parasitic oscillations resulting: from the presence of inductance in the conductors 32 and 133 are damped efiectively by providing a series circuit shunting a satficient length of the conductor and resonant at the undesired frequencyvsothat it provides a pure resistance path for current at the undesired frequency. Series resonant circuits of this type are employed in the amplifier illustrated in Fig. 1. v

it has been found that there may be substantial screen electron discharge devices are employed,

- the action of the screens avoids the necessity heating loss in the shunt resistors such as the resistances 34 and due to the passage of current through these resistances at the fundamental operating frequency of the amplifier. These heating losses may be sufllciently high to overheat the resistors and it is, therefore, desirable that the heating be minimized.

I have found that these fundamental frequency heating losses may be minimized by-providing inductances 39 and in shunt with the condensers 35 and 31, respectively, and by tuning the condensers and the respective inductances in parallel resonance at the fundamental frequency. For example, the condenser 35 and inductance 38 comprise a tuning unit which is tuned to resonance at the fundamentalfrequency and, therefore, provides a high reactance path at the fundamental frequency and minimizes the passage of current at this frequency through the resistance -34. This reduces to a minimum the heating in the resistance 34 at the fundamental frequency. It is, of course, necessary to modify the series tuning of the inductance of resistor, 34 and the capacity of the condenser 35 since the presence of the inductance 39 necessarily alters the constants of the circuit. I have found,. however, that only two or three incremental adjustmentsof the condenser 35' and inductance 35 are necessary to provide a sufficiently close approximation to simultaneous series resonance at the'undesired frequency and parallel resonance at the fundamental frequency. The inductance of the resistance 34, for example, is then in series resonance at the undesired frequency with the effective capacity of the tuning unit comprising the condenser 35 and the inductance 39.

It 'will readily be apparent that thecombination of a resistance and a tuning unitas described above may be connected in shunt to a of providing neutralizing condensers. However,

when such screen electrodes are employed at very high frequencies it is sometimes dimcult or impossible to ground them for the high frequency currents and consequently some feedback occurs to the control electrodes. reason in the circuit of Fig. 2 the neutralizing condensers 30 and 3| have been included together with the same parasitic oscillation damping circuits as are shown in Fig. 1.

Very high frequency parasitic oscillations frequently occur in amplifiers such as illustrated in Fig. 2. These oscillations may occur, for example, in a path through the plate or anode circuit of conductor 4|, the discharge space between the anode I8 and the screen grid 45,

. through the condenser 48 to ground and thence portion of any conductor in the circuit where undesired oscillations are present to damp these oscillations. For example, the damping circuit might be connected across portions of the anode or load conductors of the devices II and H indicated at 4| and 42, respectively, or across to that shown in Fig. 1 and corresponding circuit elements have been designated by the same numerals; the electron discharge devices It and II, however, are provided with screen grids or electrodes 45 and 43, respectively, conventionally located between the control electrodes and anodes of the respective tubes. These screen electrodes are employed to minimize the capacitive reaction between the anodes and control electrodes of the devices l0 and II. The screen bypass con'densers 48 and 45 being provided between the respective electrodes and ground.

In most amplifiers of the type illustratedwhen through the condenser 28 back to the .conductor 4|. A corresponding circuit, of course, is present on the .other side of the amplifier through the anode to---..- reen-grid circuit of the device H and including a portion of the plate lead or anode conductor 42.

In order to eliminate these undesired high frequency parasitic oscillations, a series combination of a resistance 50 and a tuning 1mit'including a condenser 5| and a shunt inductance 52 is connected in shunt. to the conductor 4|, and a similar series combination comprising a resistance 53 and a tuning unit including a condenser 54 and a shunt inductance 55 is connected in shunt to conductor 42. The effective capacity of the tuning unit including the condenser 5| and inductance 52 is tuned in series with the inductance'of the resistance 50 to resonate at the frequency of the undesired oscillations and .thereby presents a pure resistance path for currents at the undesired frequency. The resistance 50 may be adjusted to produce critical damping of the undesired parasitic oscillations so that such oscillations are effectively elimi- For this condenser 54, and inductance 55 is tuned in a similar manner to minimize parasitic oscillations and fundamental frequency heating in the anode I circuit of the device I It will readily be-apparent from the foregoingthat I have provided a simple and effective arrangement which makes it possible to employ a series resonant circuit for damping parasitic oscillations and for minimizing resistance heating losses in the damping circuit due to the passage of current at the fundamental operating frequency of the apparatus.

While I have shown a particularembodiment of my invention in connection with push-pull amplifier circuits other applications will readily be apparent to those skilled in the art. I do not, therefore, desire to be limited to the particular arrangements and instrumentalities shown and described and I intend by the appended claims to cover all modifications which fall within the spirit and scope of my invention. What I claim as new and desire to secure by Letters Patent the United States is:

1. Radio apparatus comprising a circuit in which oscillations of an undesired frequency tend to exist, said circuit including a conductor resonant at said undesired frequency with the effective capacity of said tuning unit effectively to damp said. oscillation, said tuning unit 'being of finite length, which thereby exhibits inducttuned in parallel resonance at the fundamental I ance across which such os illation appear, and 5 operating frequency of said amplifier for mini: a resistance and a tuning unit serially connected mizing the passage of current through said rebetween spaced points on said conductor, said i t n eatthe'fundamental frequency.

timing unit'comprising a capacitor and an inductance connected in paralle1, said resistance,

ing resonant at said undesired frequencey with the effective capacity of said tuning unit, whereby a pure resistance is presented between said spaced pointsat, said undesired frequency effectively to damp said oscillations, said tuning unit 15 being'tuned in parallel resonance at the fundamental operating frequency of said apparatus whereby passage of current through ,.saidre- 4. In a high frequency power amplifier having a triode electron (discharge device and neutralizdue to its finite length, being inductive andbe- 1Q ing means therefor, said neutralizing means in cluding a conductor of substantial lengththe inductance of which tends to resonate with other portions of said amplifier at an undesired frequency, and means in shunt to said conductor for damping oscillations of said undesired frequency, said means comprising a resistance and a tuning unit connected in series, said tuning unit comprising a, capacity and an inductance sistance at the fundamental Ire uenc is Connected parallel, Said resistance of mm q y 2 finite length and exhibiting inductance, and the 2. a high frequency power amplifier, two electron discharge devices each having an anode and a cathode and a control electrode, a circuit connected between said anode and control electrode andincluding a conductor of substantial 25 length, said circuit having a tendency to break into oscillation at an undesired frequency, said conductor due to its length having substantial inductance, and a resistance and a tuning unit serially connected in shunt to said conductor, .30 said tuning unit comprising 'a capacity and an inductance connected in parallel, said resistance being inductive due to its finite length .and being resonant at said undesired frequency with the effective capacity of said tuning unit eifec- 3 tiveLv to damp said oscillation, said tuning unit being tuned in parallel resonance at the fundamental operating frequency of said amplifier for minimizng the passage of current through said resistanceat the fundamental frequency.

3. An electron discharge amplifier including a circuit which'tends to oscillate at an undesired frequency, a portion of said circuit being a -conductor of finite length whereby it is inductive,

and a resistance and a tuning unit serially con nected in shunt to said conductor, said tuning unit comprising a capacity and an inductance connected in parallel, said resistance being inductive due to its finite length and being series 7 effective capacity of said tuning unit being of such value with respect to the inductance of said resistance as toresonate therewith at said undesired frequency effectively to damp said neutralizing means, said tuning unit being tuned in parallel resonance at the fundamental operating frequency of said amplifier for minimizing the passage of current rough said resistance at the fundamental frequency.

5. In combination. an electron discharge amplifier device having an anode and a control electrode and a screen, electrode,' a circuit through said anode and said screen electrode including a conductor of finite length which thereby exhibits inductance, said circuit tending to oscillate at an undesired frequency, and

a series combination of a resistor and a tuning unit in shunt to said conductor, said tuning unit comprising a condenser; and aninductance condo nected in parallel, said resistorhaving finite length and exhibiting, inductance and the effective-capacity of said unit being resonant therewith at said undesiredfrequency effectively to damp said circuit, said tuning unit being tuned 45 in parallel resonance, at the fundamental op-' erating frequency of said device for minimizing a the passage of current through said resistor at the fundamental frequency.

GEORGE M. BROWN. 

